A value of RhinoMath.UnsetValue indicates that the tolerance should be computed.
If the value >= 0.0, then the tolerance is set.
If the value is RhinoMath.UnsetValue, then the tolerance needs to be computed.

If the trim is not singular, then the trim must have an edge. If P is a
3d point on the edge's curve and surface(u,v) = Q is the point on the
surface that is closest to P, then there must be a parameter t in the
interval [m_t[0], m_t[1]] such that
|u - curve2d(t)[0]| <= tolerance[0]
and
|v - curve2d(t)[1]| <= tolerance[1]
If P is the 3d point for the vertex brep.m_V[m_vi[k]] and (uk,vk) is the
corresponding end of the trim's parameter space curve, then there must be
a surface parameter (u,v) such that:

the distance from the 3d point surface(u,v) to P is <= brep.m_V[m_vi[k]].m_tolerance,